If I were you, I should first look if there are (local) star trek fanclubs in your country - for instance Internet search "star trek Portugal". They will be able to tell you if there are conventions in your country - or if they are planning to do one in the future.
I know many Portuguese don't like Spain at all, but it's a bigger country and the chance of finding scifi-conventions there is higher there.
By the way : nice meeting you. Era un prazer.

Bill is right about the cost if you want a convention with big-name stars - they don't appear for free! But many fan clubs hold their own fan-run conventions. You could try getting in touch with Isaque Fernandes who is the Portuguese CO of the USS Europe, a chapter of Starfleet International.